1. Technical Field
This invention relates generally to an apparatus and process for printing onto discs, such as printing labels onto compact discs and the like.
2. Background Art
Compact discs (CDs) are high-volume data storage media having an optically-readable surface. A CD typically carries a label on the opposite side of the optically-readable surface. The label protects the CD from physical damage, and is usually printed with, say, a description of the information stored on the CD. The printed information may be logos, text, graphics, and/or bar codes. Because the CD spins at high speed in the writer or the player, the CD label needs to be precisely balanced to the center.
The labels have traditionally been printed by the screen printing technique, whenever the same image is repetitively printed on a large number of CDs. Currently, several trends exist in CD label printing, such as for example personalized labels on the labeling surface.
For personalized labels, different information is required for each CD, or for groups of a small number of CDs. The traditional screen printing method is not economical for this personalized labels because of the high cost and lengthy time involved in setting up each image.
Recently, ink jet and thermal wax transfer printing techniques have begun to be used to provide flexibility in designing and manipulating the label images. These digital printing techniques can provide flexibility in designing and manipulating the label images, and the label images can be easily designed and odified on a computer. The ink jet label printing requires a hydroscopic coating on the CD surface for accepting aqueous ink solutions. The thermal wax transfer can print on a lacquer (shiny), matte, or silk-screened disc surface. Both printing techniques, though, are bi-model in nature and are therefore not suitable for printing continues tone photographic images.
Many users want to print photographs on the labeling surface of the CDs. This is especially desired for photo CDs, where thumbnail images can be printed on the label as an index of the images stored in the CD. A photographic image is best printed with a continuous-tone printer, rather than bi-model printer such as ink jet or thermal wax transfer printers.
For artistic effect and higher information content, it is desired to print the text, graphics and images onto a disc 10 in annular-radial coordinates as illustrated in FIG. 1A, rather than in orthogonal x-y coordinates as shown in FIG. 1B. Although this may be done with screen printing, the current digital printing techniques print in orthogonal coordinates.
FIG. 2 illustrates the current CD printing method using thermal wax transfer techniques as disclosed in U.S. Pat. No. 5,317,337. The application is intended for a bi-modal (single bit) image; not for photographs. A printhead 12 is placed across the diameter of the disc and presses against the disc label surface with dye donor media 14 therebetween. During printing, the printhead moves across the outer and inner edges of the CD. This increases the wear and the probability for damage in the delicate printhead. Furthermore, since the head load is normally fixed, the nip pressure varies along the printing direction as the length of contact area changes. This causes variability in image densities and color balances, which does not allow printing at photographic qualities. Finally, this printing method prints images in an orthogonal coordinate.
U.S. Pat. No. 5,317,337 disclosed another CD label-printing technique (FIG. 3 hereof). An ink jet printhead 16 prints along the radial direction in a rectangular print area 18. For printing on a different area 20, the disc is first rotated to the desired angular position and area 20 is printed along the radial direction. Because the printing areas are rectangular in shape, they cannot perfectly match the annular shape of the label area on the CD. The alternative motions along the radial and azimuthal directions also lengthens the printing time.